Effects of Ellagic Acid on Glucose and Lipid Metabolism: A Systematic Review and Meta-Analysis

Background Abnormal glucose and lipid metabolism (GALM) serve as both a cause and an inducer for the development of the disease. Improvement and treatment of GALM are an important stage to prevent the occurrence and development of the disease. However, current clinical treatment for GALM is limited. Ellagic acid (EA), a common polyphenol present in foods, has been shown to improve abnormalities in GALM observed in patients suffering from metabolic diseases. Objective This study used a meta-analysis method to systematically assess the effects of EA on GALM. Method As of November 8, 2023, a comprehensive search was conducted across 5 databases, namely, PubMed, Embase, Web of Science, Cochrane Library, and Google Scholar to identify randomized controlled trials (RCTs) in which EA served as the primary intervention for diseases related to GALM. The risk of bias within the included studies was assessed according to the Cochrane Handbook. All statistical analyzes were performed using RevMan 5.4 software. Results In this study, a total of 482 articles were retrieved, resulting in the inclusion of 10 RCTs in the meta-analysis. The results showed that EA could reduce fasting blood glucose (FBG) (p = 0.008), increase insulin secretion (p = 0.01), improve insulin resistance index (HOMA-IR) (p = 0.003), decrease triglyceride (TG) (p = 0.004), and reduce cholesterol (Chol) (p = 0.04) and low-density lipoprotein (LDL-c) (p = 0.0004). EA had no significant effect on waist circumference (WC), body weight (BW), body mass index (BMI), 2 hours after prandial blood glucose (2 h-PG), total cholesterol (TC), and high-density lipoprotein (HDL-c). Conclusions The effect of improvement in glucose and lipids of EA was closely related to the dose and the intervention time. EA can improve GALM caused by diseases. To corroborate the findings of this study and improve the reliability of the results, EA is imperative to refine the research methodology and increase the sample size in future investigations.


Introduction
Diabetes, a major public health concern throughout the world, afects approximately 500 million people worldwide, with type 2 diabetes mellitus (T2DM) accounting for 90% of these cases [1], there is a signifcant risk to the physical and mental well-being of individuals.A study shows that nonalcoholic fatty liver disease (NAFLD), a disorder of lipid metabolism, can cause T2DM, particularly in premenopausal women, due to a further slowdown in GALM [2].Glucose and lipids are essential functional substances in the body, regulated by multiorgan hormonal feedback systems [3].Impaired metabolism of these substances can lead to metabolic dysfunction.Excessive accumulation of these substances can contribute to obesity, liver disease, hyperlipidemia, T2DM, cardiovascular disease (CVD), and other conditions [4].Furthermore, the accumulation of glucose and lipids in tumor cells can promote tumor growth and accelerate disease progression [5,6].Terefore, maintaining normal glucose and lipid homeostasis is essential for optimal health and wellness.
In recent years, the integration of herbal medicine and nutritionally rich compounds into the management of chronic diseases related to GALM such as T2DM and dyslipidemia have been considered [7][8][9][10][11][12][13]. EA, a weakly acidic polyphenol present in fruits and nuts such as blueberries, pomegranates, and walnuts, is easily converted by the body into urolithic acid for absorption [14].EA has been shown to alleviate infammation, increase the population of pancreatic beta cells, improve islet function, enhance insulin secretion, and promote serum glucose metabolism [15].EA reduces chronic infammatory responses in rats by decreasing serum levels of TNF-α and other infammatory factors [16].Furthermore, in pancreatic cells, EA mitigates microinfammatory responses by reducing TNF-α secretion, increasing cell nuclear antigen levels, and elevating IL-6 concentrations [17].EA has been found to reduce FBG levels in individuals with T2DM [18].EA upregulates gene expression of GLUT2, IGF-1, IRS-1, and IRS-2, and EA exerts antiglycation and antioxidant efects by inhibiting lipid peroxidation-mediated malondialdehyde and conjugated dienes [19,20].EA acts on α-amylase, leading to a decrease in α-glucosidase concentration [21], and inhibits glycogen decomposition by reducing glycogen phosphorylase secretion [22].EA also increases glucose uptake in L-6 cells, stimulates glycogen synthesis, and lowers serum glucose levels [23].EA can signifcantly improve the body's glucose metabolism.
EA has been found to promote lipid metabolism by reducing BW and abdominal circumference in obese rats [24].It has been demonstrated to enhance lipid metabolism by downregulating the expression of the iodothyronine deiodinase 2 and Nr4a1 genes in brown fat, while upregulating the insulin sensitizing gene PPARG coactivator 1 alpha.Tis regulatory mechanism confers an antioxidative stress efect, inhibits the generation of white fat, and improves lipid metabolism in patients with metabolic syndrome (MetS) [25].EA accelerates lipid metabolism by reducing the secretion of TG and TC by the liver of mouses [26].Furthermore, EA can decrease leptin and resistin levels in model rats, increase adiponectin secretion, inhibit adipocyte diferentiation, and enhance lipid metabolism [27].Although the specifc mechanism requires further investigation, EA has shown a protective efect against T2DM, CVD, cancer, and neurodegenerative diseases [28,29].In addition, it prevents complications of T2DM, such as diabetic retinopathy and diabetic nephropathy [30].
In conclusion, EA has the potential to increase serum GALM in metabolic diseases, thus delay disease progression.However, there remains controversy regarding the clinical use of EA [31].Te lack of clinical data on the natural compound EA and the small sample sizes in the studies are the main issues.To address this issue, a systematic review and meta-analysis of published clinical trial studies was conducted to comprehensively reassess the efects of EA on GALM.Tis analysis further clarifed the relationship between GALM and EA, elucidating the specifc contributions of EA and the relative causal efects of associated components.

Methods
Tis systematic review and meta-analysis was registered with the International Registry for Prospective Systems Evaluation (PROSPERO, CRD42023404041) according to the PRISMA checklist.Tis registration was undertaken to ensure transparency and adherence to rigorous research methodology, thereby enhancing the validity and reliability of the study.

Search
Strategies.An extensive and meticulous investigation and analysis of scholarly articles was conducted up to December 8, 2023.Tis investigation spanned several authoritative databases, including PubMed, Cochrane Library, Web of Science, Embase, and Google Scholar.Te focal point of this search was the biochemical compound EA and its potential impacts on physiological health indicators.Te selection criteria were stringent, focusing on research where key terms such as body weight, body mass index, waist circumference, fasting blood glucose, 2 hours after prandial blood glucose, hemoglobin A1c, insulin resistance index, insulin, and lipid profles (encompassing total cholesterol, cholesterol, triglycerides, and low-and high-density lipoprotein cholesterol) and were prominently featured in the research titles and abstracts.In addition, the search gives special consideration to studies exploring the levels of leptin and adiponectin, hormones that are crucial in the regulation of metabolic processes.(

Inclusion Criteria
(d) Involving RCTs published in the English language.Subsequently, they reviewed the titles and abstracts of the articles to exclude irrelevant literature.Finally, the full text of the remaining articles was downloaded and read to assess their eligibility for inclusion in the analysis.In cases of disagreement between the authors, initial discussions were held to address and resolve the discrepancies.If necessary, a third party would be consulted to assist in resolving the issue.Tis systematic approach guarantees a meticulous selection of included studies and ensures accurate and reliable analysis.

Data Extraction and Management.
Te researchers extracted relevant data from all included literature, including basic study information such as the author and date of publication, as well as subject characteristics, including the sample size, gender, age, and country of origin.Furthermore, information on intervention measures was provided, including means, frequency, and duration of treatment, as well as course of treatment.
Te primary outcome indicators, corresponding data, the follow-up time, and any reported adverse events were extracted from the studies.Two independent investigators then entered the data into standardized forms (the abovementioned steps are performed by XLW and XTZ).Disagreements were resolved through discussion and, if necessary, consultation with another author.If the information was incomplete or unclear, we contacted the original authors of the studies to obtain the necessary details.
Tis meticulous process ensures the accuracy, completeness, and reliability of the data collected for analysis, facilitating a comprehensive evaluation of the efects of EA on GALM.
2.4.3.Quality Assessment.Each study's risk of bias was meticulously assessed, referencing the guidelines outlined in the 2014 Cochrane Handbook.Tis comprehensive evaluation scrutinized seven critical aspects of the study methodology to determine potential biases.Tese aspects included the generation of a random sequence, ensuring allocation concealment, the blinding process for both participants and personnel, the blinding method applied during outcome assessment, the integrity and completeness of the outcome data reported, the presence of any selective reporting of outcomes, and the identifcation of any other possible sources of bias.
Based on the previously mentioned criteria, the quality of each study was classifed into one of the following three categories: low, high, or unknown risk of bias.Tis classifcation is instrumental in assessing the overall quality and dependability of the studies included.Furthermore, it facilitates a more accurate interpretation of the data in our systematic review and meta-analysis.

Data Synthesis and Statistical
Analysis.RevMan 5.4 software was used utilized for the meta-analysis.Te analysis involved a 95% confdence interval (95% CI) and calculation of the standard mean diference (SMD) for continuous variables.Te I 2 statistic was used to assess heterogeneity between studies.In cases where heterogeneity tests showed a p value of <0.01 and/or an I 2 value of <50%, random efects models were applied for combined efect estimates.Conversely, fxed-efects models were used for studies exhibiting low heterogeneity, as indicated by a p value >0.10 and an I 2 value <50%.To ensure the reliability of the study, we analyzed subgroups of articles with at least 10 experimental data points.We performed the subgroup analysis using RevMan 5.4 software.
Statistically signifcant was considered achieved when the p value was <0.05.In cases where the meta-analysis comprised at least 10 studies, a funnel plot analysis was performed using RevMan 5.4 software to assess publication bias.Tis assessment helps to evaluate the possible impact of publication bias on the overall results of the metaanalysis.

Result
3.1.Literature Search and Retrieval.Initially, a total of 482 articles were retrieved from major English databases, consisting of 478 articles identifed through the literature search and an additional 4 articles from other sources.After the removal of duplicates and an initial screening, 40 studies were found to be replicated and accordingly excluded from further analysis.Tis exclusion process involved the removal of 384 studies that did not meet the criteria, which included case reports, reviews, meta-analyses, noninterventional basic studies, and nonclinical controlled trials.
In this meta-analysis, 10 RCTs that fulflled the inclusion criteria were analyzed.Tey were selected based on their relevance to the research question and compliance with preestablished inclusion criteria.To visually represent the screening process, a fowchart is provided to depict the screening process visually, detailing the number of articles reviewed at each phase and the reasons for their exclusion.Te fowchart of the research and screening process is illustrated in Figure 1.Specifc search methods and search results are shown in Table 1.

Risk of Bias.
Te risk of bias in the RCTs included in the study was evaluated using the bias risk assessment criteria outlined in the Cochrane Manual.Te assessment focused on the following two aspects: the generation of randomly assigned sequences (selective bias) and allocation hiding.7 studies used the random number method, 1 study used the stratifed random method, and 2 studies used the block random method.Tis information is crucial to evaluate the potential for selective bias in included RCTs.
Regarding the concealment of the allocation, fve studies reported using sealed containers to hide random allocation schemes, suggesting a lower risk of bias.However, fve studies did not provide details on the methods used for concealment of allocation, indicating a higher risk of bias in these studies.
Figures 2 and 3 graphically represent the results of the risk of bias assessment, showing the level of bias present in the included RCTs.Tese fgures assist the reader in interpreting the overall quality and potential limitations of the studies included in the meta-analysis.Tis methodology provides a more robust and generalizable estimate of the efects of EA on metabolic indices.To fully appreciate the implications of the study fndings, it is necessary to provide the specifc results of the meta-analysis, including the efect sizes and their associated confdence intervals.Tese results could provide information on the potential benefts of the EA intervention for metabolic health and shed light on its overall impact on metabolic indices.

Main Outcome.
Due to the diferent diseases included, we performed a pooled meta-analysis of the characteristics of the baseline population included.Te results showed that there were no signifcant diferences in baseline age, BW, BMI, FBG, 2 h-PG, insulin, HOMA-IR, TG, TC, Chol, LDL-c, and HDL-c of the patients included in all studies, as shown in Table 3.
Te meta-analysis of the efects of EA on GALM following the intervention showed specifc results, as shown in Figure 4.
By deleting the included literature one by one and conducting a sensitivity analysis, it was shown that EA had a low sensitivity to WC, BW, BMI, and HDL-c and a high sensitivity to FBG, TG, TC, LDL-c, insulin, and HOMA-IR.However, the efect on Chol (p � 0.18, SMD: −0.84, 95% Cl: −2.09-0.4,and I 2 � 88%) after excluding April 2015 is unstable.Te specifc analysis results are shown in Supplementary Table S1.

Discussion
Tis meta-analysis examined the efect of EA on GALM in MetS.Te results of the meta-analysis demonstrated that EA had a remarkable efect on the reduction of FBG, TG, TC, LDL-c, insulin, and HOMA-IR.EA had no signifcant efect on abdominal circumference, BW BMI, 2 h-PG, TC, and HDL-c.In addition, the results of the sensitivity analysis demonstrated that EA has an unstable efect on cholesterol.In conclusion, EA can improve the abnormal metabolism of GALM in serum by regulating the changes in FBG, insulin, HOMA-IR, TG, and LDL-c.
Human energy intake involves the conversion of carbohydrate into glucose.Lipids, which include fats and other molecules, are crucial components of cell membranes and  8 Journal of Nutrition and Metabolism Journal of Nutrition and Metabolism serve as a primary energy source for the body [41].Research has shown that altered GALM can cause infammation, hyperglycemia, and hyperlipidemia, which ultimately results in metabolic disorders such as diabetes and hyperlipidemia [43,44].In severe cases, CVD, malignant, tumors, and neuroanatomic diseases can arise [45][46][47][48].Terefore, ensuring proper GALM is extremely important for the prevention and management of diseases.EA is a polyphenolic compound which is prevalently found in a variety of sources such as fruits grains and nuts [49].It is known for its high bioavailability in the human body [16].Recently, there has been a growing body of research focusing on EA.It showed antioxidant and antiinfammatory properties [50], protects the liver [51], enhances neuronal activity [52], and has anti-T2DM, antiatherosclerosis, and anticancer efects [53].In addition, it delays the development of chronic diseases.Research on the mechanism of glucose metabolism has shown that EA reduces serum glucose levels in rats by activating 5′AMPactivated protein kinase [54].It also mitigates glucoseinduced damage to pancreatic mesangial cells by activating the PI3K/Akt/FOXO3 signaling pathway, restoring pancreatic cell function, and reducing serum glucose concentration [20].EA improves insulin resistance in hyperuricemic rats by activating C1q/tumor necrosis factorassociated protein-3 and inhibiting ATP citrate lyase, thereby increasing glucose metabolism [55].EA reduces serum glucose levels by decreasing advanced glycation end products and increasing glucose utilization [56].It inhibits the expression of glucose-α-glucosidase activity, leading to a decrease in serum glucose concentration [20].Furthermore, EA acts as an antioxidant by regulating the expression of peroxisome proliferator-activated receptors-c and insulin receptor subunit-1.Tis improves systemic infammation in white adipose tissue, increases lipid metabolism, improves insulin sensitivity, and reduces serum glucose concentration.Furthermore, EA can prevent the death of beta cells, increase their quantity, and improve their function by decreasing the expression of the insulinsensitive gene caspase 3 [57].Tis results in elevated insulin secretion and an overall antidiabetic efect.Te effectiveness of EA in reducing glucose levels is comparable to  [44,58].
In general, studies indicate that EA exerts benefcial efects on lipid metabolism [59].EA increases the uptake of LDL-c and the secretion of the apoA-1 apolipoprotein, while maintaining HDL-c within the normal range [60].Administration of EA in mice resulted in a reduction in FBG levels and an increase in serum adiponectin [31].In addition, EA reduces the concentration of TG and TC secreted by the liver, thus enhancing lipid metabolism [24].Tis is achieved by enhancing the expression of the peroxisome proliferatoractivated receptor-c and CCAAT/enhancer binding protein-α protein and promoting EA oxidation [61].EA decreased TG and Chol levels and improved liver lipid metabolism in aged liver by upregulating the expression of silent information regulator 1, adenosine 5-monophosphateactivated protein kinase, sterol regulatory element binding protein 1, peroxisome proliferator-activated receptors, and phosphorylated AMPK in the liver [62].Te meta-analysis found that EA efectively reduces serum TG, Chol, and LDL-c concentrations and improves lipid metabolism in patients.However, EA was found to be less efective in afecting HDL-c levels.Subgroup analysis suggests that this can be attributed to the diferent types of disease included in the studies.
Te results of the meta-analysis indicate that EA supplementation does not result in a signifcant change in WC, BW, TC, and HDL-c levels in patients.However, subgroup analysis suggests that EA can reduce BMI, TC, and HDL-c levels in patients after correlation with RCTs but does not lead to signifcant changes in WC and BW.Te degree of improvement in insulin resistance was found to be related to the dose and duration of the EA intervention.In addition, no adverse reactions were reported in these clinical trials.Tese fndings suggest that EA supplementation has a signifcant impact on improving serum GALM in patients.Te evidence indicates that EA has a positive impact on serum GALM.4.1.Limitations.Tis systematic review and meta-analysis have several limitations that should be acknowledged.One major limitation is the signifcant heterogeneity observed in most of the pooled outcomes.Subgroup analyzes, stratifed by age, dose, and duration of administration, failed to substantially mitigate heterogeneity in pooled results for most of the targeted endpoints.However, it should be noted that the stability of BMI, FBG, TG, TC, HDL-c, insulin, and HOMA-IR results can be infuenced by the dose and duration of administration, which requires further investigation.
In addition, the low methodological quality of the included studies may have slightly reduced the reliability of the combined results.Te small sample size of articles analyzed in this study makes it challenging to establish defnitive conclusions.Te literature reviewed was limited to publications in English, which could restrict generalizability and EA supplementation to ethnic groups whose primary language is not English.Based on the fndings of this study, well-designed, rigorous RCTs with larger populations are needed to investigate the role of EA and the relationship between dose, duration of the intervention, therapeutic efcacy, and adverse reactions.Furthermore, for the management of patients with elevated serum glucose and lipid levels, a focus on long-term patient care is essential.To date, there have been limited clinical investigations on oral efects of EA in such patients, and future large-scale, well-designed trials will be necessary to confrm the therapeutic mechanism of EA.

Conclusions
In conclusion, our fndings indicate that EA supplementation can efectively lower serum glucose levels and improve lipid metabolism in patients, indicating potential benefts for the management of abnormal GALM.Tis systematic review sheds light on the role of EA supplementation in GALM.We hope that this study will provide clinicians with additional options to incorporate EA into their therapeutic strategies.
(a) Te design is a randomised controlled trial (RCT) (cross design or parallel design).(b) EA or placebo was the primary intervention.Te treatment group was treated with EA as the intervention method, while the control group was treated with placebo.(c) Te main outcome index data extracted are waist circumference (WC), body weight (BW), body mass index (BMI), fasting blood glucose (FBG), 2 hours after prandial blood glucose (2 h-PG), insulin, insulin resistance index (HOMA-IR), triglyceride (TG), total cholesterol (TC), cholesterol (Chol), low-density lipoprotein (LDL-c), high-density lipoprotein (HDLc), leptin, and adiponectin.Outcome index BMI and HOMA-IR calculation formulas are as follows: BMI kg/m 2   � Body weight Height2 , HOMA − IR � Fasting blood glucose * Fasting insulin 22. 5 .

( a )
Clinical studies of non-EA interventions.(b) Publications that lack a diversity of essential data, including thematic information or interventions.(c) Te results of the study could not be verifed or analyzed.(d) Duplicate publications.

Table 1 :
Search methods of meta-analysis.
3.4.Meta-Analysis.Te meta-analysis incorporated 10 RCTs involving a total of 419 patients.Random-efects models were used to analyze the efects of EA on various metabolic indices before and after the intervention.Tis approach allows for the summarization of the results while taking into account potential heterogeneity among the included studies.

Table 2 :
Characteristics of included studies.
Journal of Nutrition and Metabolismand HDL-c (p � 0.13, SMD: 0.32, 95% Cl: −0.09-0.72,andI 2 � 68%).In conclusion, EA did not lead to a reduction in WC, BW, and BMI.EA improved FBG levels, but it did not afect 2h-PG levels.EA can increase insulin secretion, reduce HOMA-IR, and improve insulin resistance in patients.EA can reduce the levels of TG, Chol, and LDL-c, but it has no signifcant efect on TC and HDL-c.Te funnel plots are shown in Supplementary FigureS1.

Table 3 :
Baseline analysis of meta-analysis.

Table 4 :
Subgroup analysis of meta-analysis.